1. Field of the Invention
The present invention relates to apparatus and methods for rinsing and drying semiconductor wafers and other semiconductor substrates that are used to fabricate semiconductor dies, which are also known as integrated circuit dies. Particularly, the present invention relates to apparatus for rinsing and drying semiconductor wafers without requiring physical transfer of the individual wafers between locations between the rinsing and drying steps. More particularly, the present invention relates to apparatus and methods for rinsing and drying wafers which utilize a drying compound. A preferred apparatus according to the invention evacuates water slowly from the semiconductor wafers during drying of the wafers. A preferred drying compound for use in the apparatus and method of the present invention is a non-heated isopropyl alcohol (IPA) vapor. The present invention also relates to a chamber in which wafers are rinsed and dried.
2. Background of Related Art
A continuing goal of integrated circuit processing is to produce wafers having ever-fewer and ever-smaller particles of contamination per semiconductor wafer. Another goal of integrated circuit processing is to clean the ever-thinner, ever-larger (i.e., which are increasing from 6 to 8 to 10 inches in diameter), state-of-the-art semiconductor wafers without breaking or otherwise damaging them. Several processes and apparatus have been developed for removing different types of contamination from the surfaces of semiconductor wafers, including without limitation silicon, gallium arsenide, silicon-on-glass (SOG), silicon on sapphire (SOS), silicon on insulator (SOI), and other semiconductor wafers known in the art. Many such processes involve the use of chemicals. The need for atomically clean surfaces requires the complete rinsing of all chemicals and debris from the wafer surface. Clean wafers are essential at all stages of the fabrication process but are especially necessary before any of the operations performed at high temperature.
Several rinsing and drying methods are known and used in the relevant industry. One such method is known as the "quick dump" method, wherein the wafers are placed into a rinse tank and submerged in de-ionized (DI) water or any other suitable rinse liquid to wash solvent, rinse acids and/or rinse bases, and contaminants from the surface of the wafers. Some dump rinsers spray the water onto the wafers. The water is then quickly removed from the rinse tank. Other dump rinsers have a flow meter, which controls the rate at which water exits the chambers. The rinse-dump process may be repeated several times in order to further decrease the number of contaminants on the wafers.
Even with repetitive cycles, the "quick dump" method often leaves particles on the wafer, making that method undesirable for many applications. This is principally due to the rapid deployment of water from the rinse tank, which often transfers many of the contaminants back onto the wafer. The quick discharge of water from the rinse tank and the resultant quick flow of air into the tank also create friction with the surface of the wafers, which tends to generate an electrostatic charge on the surfaces of the wafers, increasing their tendency to attract contaminants. The number of contaminant particles often correlates to lower die yields on the semiconductor wafer. Further, many quick dump rinsers are not equipped to dry the wafers, which requires physical transfer of the wafers to a separate device and increases the cleaning time and cost of the finished product. Use of separate rinsing and drying devices may also increase the possibility of additional contaminants adhering to the wafers.
Devices known as overflow washers have also been used in the industry. Overflow washers include a rinse tank which has a continuous supply of a rinse liquid, such as DI water. The rinse liquid flows into many overflow rinsers through the bottom of the rinse tank. As the tank fills with fresh, clean water, dirtier water is removed from the upper portion of the rinse tank. Many overflow washers also include a bubbler, which introduces a stream of nitrogen bubbles into the bottom of the rinse tank to enhance the rinsing action of the flowing water. As the nitrogen bubbles move up through the water and past the wafers, the bubbles facilitate the mixing of the chemicals on the wafer surface with the rinse liquid.
Many overflow washers employ resistivity meters to determine the duration of the rinsing process. Such meters determine the resistivity of the rinse liquid that exits the rinse tank. Chemicals which were used to clean the wafers dissolve into the rinse liquid and act as charged particles. In this fashion, the resistivity meter detects the presence of chemicals based on the resistivity of the rinse liquid. For example, if the rinse liquid entering the rinser has a resistivity of 18 mega ohms, a reading of 15 to 18 mega ohms on the exit side indicates that the wafers are cleaned and rinsed.
Due to its utilization of large volumes of rinse liquid, overflow rinsing is perceived as undesirable to many in the industry. Further, many overflow rinsers are not capable of drying the wafers. Thus, drying requires movement of the wafers to a dryer, which increases the cleaning time, processing cost, and the possibility that additional contaminants will adhere to the surface of the wafers. Due to these shortcomings, many overflow rinsers do not adequately reduce the level of contamination on the wafer surfaces.
Another type of rinsing apparatus which has found widespread use in the industry is the cascade washer, which includes a series of adjacent overflow washers. In use, fresh rinse liquid flows into the first, highest washer of the series. As water fills the first overflow washer and then discharges, it enters the second, which fills and then discharges into the third, and so forth. Wafers are first placed in the last washer of the series, which has the most contaminated rinse liquid supply from the cleaning of one or more preceding wafers or sets of wafers. The wafers are then sequentially repositioned into each adjacent washer until they are eventually washed in the first overflow washer, which has the freshest and cleanest water supply.
The effectiveness of many cascade rinsers is often limited because "dirty water" may be present in the first chamber. The dirty water typically includes residual acid and particles from preceding wafers or sets of wafers. Some of the residual particles tend to attach to the wafer, which can cause defects in an integrated circuit fabricated thereon, thereby reducing the number of good dies on a typical wafer. As noted above, cascade rinsers also require repeated movement of the wafers from chamber to chamber, which increases the cleaning time. Generally, cascade rinsers lack a drying mechanism, making such rinsers further undesirable due to the aforementioned contamination problems associated with handling of the wafers. The wafers must also be physically transferred to a drying apparatus. Subsequent drying operations may also introduce more particles onto the wafers, which may decrease the number of good dies on each of the wafers.
U.S. Pat. No. 5,635,053, issued in the names of Aoki et al. (the "'053 Patent"), discloses another method and apparatus for rinsing wafers. The method of the '053 Patent involves rinsing with an anolyte or a catholyte electrolytic ionized water (EIW) which has been produced from DI water. The pH of the EIW is selected based upon the cleaning agents used on the wafers in preceding cleaning steps.
The rinsing method of the '053 Patent is undesirable due to the additional costs associated with electrolytically ionizing water. In addition, the apparatus described in the '053 patent does not dry the wafers, introducing the potential problems identified above with other devices which do not rinse and dry wafers.
After the wafers have been rinsed, they must be dried. The presence of any amount of residual water on the surface of a wafer (even atoms) has the potential of interfering with each subsequent operation. Known methods for drying wafers include spin rinse drying, the use of hot water, and the use of drying chemicals.
Spin rinse dryers have been used in the art to dry wafers or to rinse and dry wafers. Spin rinse dryers employ a combination of rinse liquid spray and centrifugal force to remove contaminants and rinse liquid from the wafer. Many spin rinse dryers also spray hot nitrogen gas onto the wafers during spinning. U.S. Pat. No. 5,022,419, issued to Thompson et al., discloses an automated spin rinse dryer with a removable heated chamber bowl which is preferably mounted at an angle to facilitate the gravitational flow of effluent drainage from the chamber. That device also includes a broken chip collector, an acidity sensor, and an exhaust manifold assembly.
Many spin rinse dryers tend to generate undesirable amounts of static charge during their spin cycles. Some of the static charge collects on the surfaces of the wafers, attracting contaminants thereto. Many spin rinse dryers also tend to damage or break the delicate larger and/or thinner wafers that are presently being used in the industry.
Another drying technique which is used in the industry includes the use of hot DI water. Hot DI water evaporates more quickly than DI water at ambient temperatures (i.e., room temperature DI water). However, the use of hot DI water is undesirable in that it may introduce stains on the wafer. Additionally, hot DI water is an aggressive solvent that often deteriorates equipment, thereby increasing maintenance operation costs. Heating the DI water also adds to the cost of cleaning the wafers.
Processes and apparatus which utilize drying fluids are also used in the industry for drying the rinse liquids from wafers. Conventional isopropyl alcohol (IPA) mist or fog systems and full displacement systems are capable of producing a wafer having a very dry surface finish, a very low particle contamination count, and little or no electrostatic charging of the wafers. In many IPA systems, the wafer must be transferred from a separate rinse apparatus to a self-contained drying module. Many IPA drying apparatus direct a pressurized IPA stream to a heated plate in order to produce a hot IPA mist or fog which dries the wafer. When wafers are dried in systems which utilize cleaning fluids, little or no electrostatic charge develops on the wafers.
The following U.S. Patents disclose methods for drying semiconductor wafers using drying fluids: No. 5,653,045, issued to Ferrell (the "'045 Patent"); No. 5,634,978, issued to Mohindra et al. (the "'978 Patent"); and No. 5,571,337, issued to Mohindra et al. (the "'337 Patent"). The '045 Patent discloses a method and apparatus for drying semiconductor wafers with a mist or fog of drying fluid. The '978 and '337 Patents each disclose a method and apparatus in which wafers are immersed in a water-containing liquid, the water-containing liquid is displaced with a gaseous mixture, and a drying fluid is pulsed at the edges of the wafers to remove liquid therefrom.
However, IPA and many other cleaning fluids are highly flammable. Thus, special safety precautions are necessary to avoid fires when heating such cleaning fluids. Some such dryers utilize large quantities of drying fluids, which can pose health and environmental hazards. Additionally, large quantities of some hot solvents can be incompatible with certain resist patterned wafers.
What is needed is a method and apparatus for rinsing and drying semiconductor wafers that effectively reduces levels of contaminants remaining on the wafers. A wafer-cleaning method and apparatus that create little or no static charge on the wafers and that adequately clean relatively large and thin, state-of-the-art wafers without tending to damage or break the wafers are needed. There is a further need for a method and apparatus that eliminate the need for transferring wafers between rinsing and drying steps, reduce the amounts of rinse liquids and drying fluids consumed, and otherwise reduce costs associated with cleaning wafers.